Dialkyl carbonates are important intermediates for the synthesis of fine chemicals, pharmaceuticals and plastics and are useful as synthetic lubricants, solvents, plasticizers and monomers for organic glass and various polymers, including polycarbonate, a polymer known for its wide range of uses based upon its characteristics of transparency, shock resistance and processability.
One method for the production of polycarbonate resin employs phosgene and bisphenol-A as starting materials. However, this method has numerous drawbacks, including the production of corrosive by-products and safety concerns attributable to the use of the highly toxic phosgene. As such, polycarbonate manufacturers have developed non-phosgene methods for polycarbonate production, such as reacting diphenyl carbonate with bisphenol-A.
Dimethyl carbonate has a low toxicity and can also be used to replace toxic intermediates, such as phosgene and dimethyl sulfate, in many reactions, such as the preparation of urethanes and isocyanates, the quaternation of amines and the methylation of phenol or naphthols. Moreover, it is not corrosive and it will not produce environmentally damaging by-products. Dimethyl carbonate is also a valuable commercial product finding utility as an organic solvent, an additive for fuels, and in the production of other alkyl and aryl carbonates.
Dimethyl carbonate, as well as other dialkyl carbonates, have traditionally been produced by reacting alcohols with phosgene. These methods have the same problems as methods that use phosgene and bisphenol-A, i.e. the problems of handling phosgene and disposing of phosgene waste materials. Thus, there is a need for commercially viable non-phosgene methods for the production of dimethyl carbonate, as well as other dialkyl carbonates.
Non-phosgene methods that have been proposed for producing dialkyl carbonates include the transesterification reaction of alcohols and cyclic carbonates. Most of the proposed methods relate to the use of various catalysts for that reaction. Examples of such proposed catalysts include alkali metals or basic compounds containing alkali metals; tertiary aliphatic amines; thallium compounds; tin alkoxides; alkoxides of zinc, aluminum and titanium; a mixture of a Lewis acid and a nitrogen-containing organic base; phosphine compounds; quaternary phosphonium salts; cyclic amidines; compounds of zirconium, titanium and tin; a quaternary ammonium group-containing strongly basic anion-exchange solid material; a solid catalyst selected from the group consisting of a tertiary amine-or quaternary ammonium group-containing ion-exchange resin, a strongly acidic or a weakly acidic ion-exchange resin, a mixture of an alkali metal with silica, a silicate of an alkaline earth metal and an ammonium ion-exchanged zeolite; and a homogeneous carbonation catalyst selected from the group consisting of tertiary phosphine, tertiary arsine, tertiary stibine, a divalent sulfur compound and a selenium compound.
With respect to the method for carrying out the above-mentioned transesterification reaction between a cyclic carbonate and an alcohol, the proposed methods generally rely upon using commercial grade reactants (i.e., the cyclic carbonate and the alcohol) having a relatively high purity to achieve adequate reaction rate, yield and selectivity to the desired dialkyl carbonates and diols, as discussed more fully below.
It is known to react alkylene oxides with CO2 in the presence of catalysts to give cyclic carbonates. The proposed methods for this reaction again mostly relate to the development of various catalysts. Examples of such proposed catalysts include alkali metal halides; ammonium, phosphonium or sulphonium halide carbonates; a combination of protic compounds, such as alcohols, and nitrogen-containing bases; arsonium halides; tertiary phosphines with alcohols; and alkali metal transfer catalysts with crown ethers and other ligands.
Although some of these proposed methods for producing cyclic carbonates will provide relatively high yields and selectivity to the desired cyclic carbonate, inevitably a significant amount of glycols are produced as by-products of the reaction between the alkylene oxides and CO2. For example, the reaction between ethylene oxide and CO2 to produce ethylene carbonate will inevitably produce a certain amount of ethylene glycol and higher molecular glycols, e.g., di- and tri-ethylene glycol. It is generally believed that the presence of these glycol impurities contained in the cyclic carbonate will adversely affect the transesterification reaction between the cyclic carbonate and an alcohol, since certain glycols, e.g., ethylene glycol, will typically unfavorably affect the equilibrium of the reaction, thereby lowering yield or possibly selectivity for the desired products. In addition, the glycols have hydroxyl (—OH) groups which would be expected to compete with the alcohol in reacting with the cyclic carbonate, forming unwanted heavier carbonates and other undesired species. Moreover, since the glycol impurities, e.g., ethylene glycol and triethylene glycol, are generally hygroscopic in nature, the cyclic carbonate containing these glycol impurities will tend to absorb water during storage. When water is present in sufficient quantity in the reaction mixture, hydrolysis takes place simultaneously with the transesterification reaction, resulting in a decrease in the selectivity for the dialkyl carbonate. Thus, in order to avoid the problems associated with the glycol impurities, the proposed methods for producing dialkyl carbonates from a cyclic carbonate have required a relatively high purity cyclic carbonate.
In order to provide a high purity commercial grade cyclic carbonate, e.g., ethylene carbonate, useful as a reactant in connection with the proposed methods discussed above, difficult separations have to be performed to achieve the requisite cyclic carbonate purity, resulting in increased operating and capital costs. Typically, the purification process for a cyclic carbonate, e.g., ethylene glycol, produced by reacting an alkylene oxide with CO2, will include the following: (1) flashing the carbonation reactor effluent to remove the light ends; (2) passing the remaining liquid through one or more evaporators to remove and recycle catalyst, and to remove heavies; (3) distilling the crude product stream containing the cyclic carbonate and mono- and poly-glycols in a first vacuum distillation column, containing between about 8 and 50 trays, to remove the majority of the mono- and poly-glycols; and (4) distilling the purified cyclic carbonate stream in a finishing vacuum distillation column, containing between about 8 and 50 trays, to remove remaining polyglycols and other heavies and to provide a high purity cyclic carbonate.
There are significant capital and operating costs associated with the two vacuum distillation columns needed to provide a high purity cyclic carbonate. Moreover, a significant amount of the cyclic carbonate is generally lost as a result of the purification process. For example, in the case of ethylene carbonate, an azeotrope is formed in the first vacuum distillation column, which can contain about 91% ethylene glycol and 9% ethylene carbonate, depending upon the pressure in the column, so that a significant amount of ethylene carbonate is removed with the ethylene glycol. Further, depending upon the operating conditions of the distillation columns, and specifically the high temperatures associated with full reboiled columns, a certain amount of decomposition of the cyclic carbonate can occur resulting in additional losses. Moreover, additional cyclic carbonate is lost in connection with recycling the homogeneous carbonation catalyst and removing heavies from that catalyst recycle stream.
One patent that discloses a process for producing a dialkyl carbonate, which does not require a highly purified cyclic carbonate is U.S. Pat. No. 5,218,135 (Buysch et al.). The Buysch et al. patent discloses a process for the preparation of dialkyl carbonates from alkylene oxides, CO2 and alkanols, where an alkylene carbonate is formed in a first step by reacting an alkylene oxide with CO2 and then reacting the alkylene carbonate, formed in the first step, with an alkanol to form a dialkyl carbonate. It further discloses that the first step reaction is conducted in the presence of a bifunctional halide/lewis acid catalyst at pressures below 10 bar and the second reaction is conducted in the presence of the same bifunctional halide/lewis acid catalyst under autogenous pressure. However, there are problems associated with using this process on an industrial scale. The problems include long reaction times for the first step reaction between the alkylene oxide and CO2, high corrosivity of many Lewis acids, thus making the process unattractive for use on an industrial scale.
As can be understood from the above, no proposal has heretofore been made with respect to the present invention's integrated process for producing a dialkyl carbonate and a diol from an alkylene oxide, carbon dioxide and an alcohol, having high productivity, which first reacts an alkylene oxide with carbon dioxide in the presence of a homogeneous carbonation catalyst to provide a cyclic carbonate and then feeds both the cyclic carbonate and the homogeneous carbonation catalyst to a second, transesterification reactor containing a heterogeneous transesterification catalyst. Additionally, no proposal has heretofore been made with respect to the present invention's integrated process useful on an industrial scale for producing a dialkyl carbonate and a diol from an alkylene oxide, carbon dioxide and an alcohol, having high productivity, which first reacts an alkylene oxide with carbon dioxide in the presence of a homogeneous carbonation catalyst to provide a cyclic carbonate and then feeds both the cyclic carbonate and the homogeneous carbonation catalyst to a second reaction zone to react the cyclic carbonate with an alcohol to provide a dialkyl carbonate and diol having relatively high conversion rates with high selectivity to the desired products.